1. Field of the Invention
The present invention concerns a process for producing polyurethane compositions with a low content of diisocyanate monomer(s), the polyurethane compositions that can be produced by this process, and their use, especially in bonding processes (e.g., in hot-melt bonding processes) or for coating purposes. The invention also concerns adhesives and coating compounds that contain polyurethane compositions of this type.
2. Description of the Related Art
Many different processes for producing polyurethane compositions are known. However, these processes usually involve more than two process steps. The more complex the process and the greater the number of components, the more expensive and susceptible to error the process becomes.
There are a huge number of polyurethane compositions. In industrial applications, they are generally adjusted to specific applications according to user specifications, and in this regard, especially moisture-reactive polyurethane compositions are used as hot-melt adhesives.
However, the use of these polyurethane compositions is not without its problems, because the diisocyanate monomers they contain are toxic, injurious to health, irritating, and sensitizing. This means that during the processing of polyurethane compositions, care must be taken to ensure that the maximum allowable concentration (MAC) of the isocyanates, which depends on their vapor pressures, is not exceeded. The MAC Commission has even classified various monomeric isocyanates in Category 3 “Substances with Well-Founded Suspicion of Carcinogenic Potential” of Section III (Carcinogenic Workplace Substances) of the MAC List. The Trade Association of the German Chemical Industry published a special code of practice entitled “Polyurethane Production and Processing/Isocyanates” with information on the safe handling of isocyanates. According to this publication, the handling of monomeric diisocyanates and compositions that contain monomeric diisocyanates requires special care, and according to the provisions of the Hazardous Substances Act, monomeric diisocyanates and compositions that contain diisocyanates must be labeled accordingly. However, when the content of monomeric diisocyanates in the composition is very low (less than 0.1 wt. %), there may no longer be an obligation to provide hazardous substance labeling.
DE 199 57 351 A1 describes the production of a reactive polyurethane adhesive/sealant composition with a low content of isocyanate monomers, in which a diol component is reacted with a monomeric diisocyanate, the unreacted monomeric diisocyanate is removed from the reaction product by recrystallization and/or washing with an aprotic solvent, and the high-molecular-weight diisocyanate formed in this way is reacted with a polyol. This process has the disadvantage that the excess monomeric diisocyanate must be removed to obtain a low content of monomeric diisocyanate in the polyurethane composition.
DE 199 31 997 A1 describes a polyurethane binder with a low content of readily volatile residual monomers and improved slip agent adhesion and a multistep process for producing it. Various difunctional isocyanates and polyol components are used in the multistep process. This makes the process very expensive and subject to error, since the various components must be used in the production process at precisely the right times and in precisely the right amounts.
Therefore, one of the goals of the invention is to develop a process for producing a polyurethane composition with a low content of diisocyanate monomer(s).
Another goal of the present invention is to make available a polyurethane composition with a low content of diisocyanate monomer(s), which can be used in a bonding process or for coating purposes.